Propionibacterial cell for organic acid production

ABSTRACT

Microbial cell lines suitable for industrial-scale production of organic acids and methods of making and isolating such cell lines.

CLAIM OF PRIORITY

This application is a continuation application of and claims priority under 35 U.S.C. § 120 to U.S. application Ser. No. 16/443,554, filed on Jun. 17, 2019, which will issue as U.S. Pat. No. 10,808,266, which in turn claims the benefit under 35 USC § 119(e) to U.S. Patent Application Ser. No. 62/686,463, filed on Jun. 18, 2018, the entire contents of each of which are incorporated herein by reference.

SEQUENCE LISTING

The instant application contains a Sequence Listing which has been submitted electronically in ASCII format and is hereby incorporated by reference in its entirety. Said ASCII copy, created on Oct. 19, 2020, is named 37942-0022002seq.txt and is 102,522 bytes in size.

TECHNICAL FIELD

The disclosure generally relates to microbial cell lines that overproduce organic acid and methods of making the same.

BACKGROUND

Organic acids refer to carbon-containing compounds having acidic properties. Examples of organic acids include acetic acid, citric acid, gluconic acid, lactic acid, propionic acid, among many others. Because they are fully degradable, organic acids can be used in the production of biodegradable polymers. They also have other important industrial applications, including as food additives.

SUMMARY

The disclosure provides microbial cell lines suitable for industrial-scale production of organic acids and methods of making and isolating such cell lines.

In one aspect, a method of making and isolating a microbial cell line is provided, where the isolated microbial cell line overproduces an organic acid compared to the parental microbial cell line. The method uses serial passage of a parental strain in pH-controlled culture media supplemented with the organic acid, preferably in non-immobilized culture, where the pH is controlled at a value above the pKa value of the organic acid. In some embodiments, the pH is preferably in the range between about 5.5 and about 7.5, more preferably at or near neutral, between about 6.0 and about 7.0, and most preferably at about 7.0. In some embodiments, the culture media is solidified. In some embodiments, the culture medium is supplemented with the organic acid in an amount sufficient to inhibit normal microbial cell growth, e.g., to reduce doubling rate or growth rate, e.g., by at least 5%, 10%, 20%, 30%, 40%, 50%, or more. In some embodiments, the organic acid is supplemented at a progressively increasing amount in successive iterations of the serial passage. In some embodiments, the organic acid is supplemented at the same amount in successive iterations of the serial passage. In some embodiments, the organic acid is propionic acid, lactic acid, acetic acid, or butyric acid. In some embodiments, the organic acid is propionic acid, e.g., the culture media is supplemented with about 1.0%-3.0% of propionic acid, e.g., about 3.0% of propionic acid. In some embodiments, the parental cell line is a wild-type organism. In some embodiments, the parental cell line is a microbial cell line is derived from unicellular microbes.

In another aspect, a microbial cell line that overproduces an organic acid is provided, where the microbial cell line is made and isolated using serial passage in pH-controlled culture media supplemented with the organic acid, where the pH is controlled at a value above the pKa value of the organic acid, preferably in the range between about 5.5 and about 7.5, more preferably at or near neutral, between about 6.0 and about 7.0, and most preferably at about 7.0.

In another aspect, a microbial cell line that overproduces an organic acid is provided, where the microbial cell line has mutations that primarily alter, directly or indirectly, the structure, composition, and/or function of the cellular envelope. Preferably, the microbial cell line includes at least 2 genome mutations identified in Table 3 or analogous mutations. More preferably, the microbial cell line includes all of the genome mutations identified in Table 3 or homologous mutations. In one embodiment, the microbial cell line includes mutations in at least 2 genes identified in Table 3 or analogous mutations thereto. In another embodiment, the microbial cell line includes mutations in all of the genes identified in Table 3 or their homologs (e.g., homolgous genes in another species described herein). In another embodiment, the microbial cell line includes a mutation in O-antigen ligase domain-containing protein. In another embodiment, the microbial cell line includes a mutation in M18 family aminopeptidase. In another embodiment, the microbial cell line includes a mutation in amino acid permease. In another embodiment, the microbial cell line includes a mutation in adenine glycosylase.

The microbe can be any microbe that produces an organic acid. In one embodiment, the microbe is from the genus Propionibacterium (Acidipropionibacterium), and more preferably the species P. acidipropionici. In another embodiment, the microbe is from the genus Lactobacillus, and more preferably the species L. acidophilus. In another embodiment, the microbe is from the genus Acetobacter. In another embodiment, the microbe is from the genus Gluconobacter. In another embodiment, the microbe is from the genus Clostridium, and more preferably the species C. butyricum. In some embodiments, the organic acid is propionic acid. In some embodiments, the organic acid is lactic acid. In some embodiments, the organic acid is acetic acid. In some embodiments, the organic acid is butyric acid.

Also provided herein are methods of producing organic acids using the methods and microbes described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows growth of wild-type and mutant P. acidipropionici on solid buffered medium with or without addition of 1.0% PA, showing the phenotype of Strain 3-1.

FIG. 2 shows production of PA by a mutant strain of P. acidipropionici (Strain 3-1) relative to wild-type in bioreactors using bleached American Beauty Cake Flour (WFM as bolus). 1:200 inoculation, 1 L working culture, 5% (w/v) glucose equivalent WFM, 30° C., pH 7 (NaOH, 5M).

DETAILED DESCRIPTION

The most common organic acids are carboxylic acids, whose acidity is associated with the carboxyl group (—COOH). They are generally weak acids with pKa values between about 4-5. Propionic acid (“PA”), for example, is a carboxylic acid with the chemical formula C₂H₅COOH or C₃H₆O₂. It is a colorless, oily, and pungent (think Swiss cheese and sweat) liquid and has physical properties between those of the smaller carboxylic, formic, and acetic acids, and the larger fatty acids. It has a molecular weight of about 74.1 g/mol, and a pKa of about 4.9, which means in a solution having a pH of about 4.9, half of the PA is in the protonated (or undissociated), uncharged state (C₂H₅COOH), while the other half is in the deprotonated (or dissociated), negatively charged state (C₂H₅COO⁻), known as propionate or propanoate ion, which can form salt or ester compounds. As the pH decreases (becoming more acidic), more PA is in the protonated, uncharged state; when the pH increases (becoming more basic), more PA is in the deprotonated, negatively charged state.

Because PA inhibits the growth of mold and some bacteria at levels between 0.1 and 1% (w/v), PA and its salts are used as a preservative in both animal feed and human food (such as baked goods). In the United States, PA is “generally recognized as safe,” or GRAS, by the Food and Drug Administration when used as a food additive. It is also approved for use as a food additive in Australia, New Zealand, and the EU. In addition, PA is an important intermediate in the synthesis of other chemicals, such as cellulose-derived plastics, pesticides, fruit flavors, perfume bases, and pharmaceuticals.

While they are widely distributed in nature, commercial production of organic acids has generally relied on chemical synthesis because it is more economically competitive. For example, PA is currently commercially produced almost exclusively through petrochemical processes. As prices for crude oil and petrochemicals increase, along with the rapid development in the biotechnology field, the economic gap between manufacturing costs of PA via chemical synthesis and via microbial fermentation is narrowing. Coupled with growing concerns about energy shortages and environmental pollution, there has been an increasing interest in commercial-scale biosynthesis of organic acids such as PA from renewable resources.

Microbial production of organic acids by fermentation has been known and used for centuries. For example, Aspergillus niger and Yarrowia lipolytica have been used to produce citric acid; Lactobacillus has been used to produce lactic acid; Clostridium has been used to produce acetic acid; Aspergillus niger and Gluconobacter have been used to produce gluconic acid.

Propionibacterium is the microorganism most often used in the production of PA (as well as vitamin B₁₂ and Swiss cheese). Propionibacterium is a gram-positive, non-motile, non-spore forming, rod-shaped, anaerobic genus of bacteria that includes the species P. freudenreichii, P. acidifaciens, P. cyclohexanicum, P. australiense, P. acidipropionici, P. jensenii, P. thoenii, P. microaerophilum, P. olivae, P. damnosum, P. propionicum, P. acnes, P. avidum, P. granulosum, P. humerusii, and P. lymphophilum. For industrial PA production, the most commonly used strain is P. acidipropionici. (A proposal has been made to reclassify the species within the genus Propionibacterium into three novel genera: Acidipropionibacterium, Cutibacterium, and Pseudopropionibacterium (Scholz & Kilian 2016). However, Propionibacterium acidipropionci and Acidipropionibacterium acidipropionici are still used somewhat interchangeably.) The optimal pH and temperature for Propionibacterium cell growth are about 6.0-7.0 and about 30-37° C., respectively (Ahmadi et al. 2017). Cell growth is inhibited in pH less than about 5.0, although fermenters started at neutral pH can reach pH 4.4 (Rehberger and Glatz 1998). Ahmadi et al. provides an overview of PA production on several carbon sources by various species of Propionibacterium as reported in the literature (Ahmadi et al. 2017) and is incorporated herein by reference.

PA can also be produced by other anaerobic bacteria, such as certain species of Anaerovibrio, Bacteroides, Clostridium, Fusobacterium, Megasphaera, Propionispira, Selenomonas, and Veillonella.

There are a number of fermentation pathways that convert carbon sources to PA through a series of enzymatic reactions. The primary fermentation pathway involved in PA production, especially in propionibacteria, is known as the Wood-Werkman cycle, which produces propionate from pyruvate, the terminal product from glycolysis, and involves many intermediates, including oxaloacetate, malate, fumarate, succinate, succinyl-CoA, methylmalonyl-CoA, and propionyl-CoA, and many enzymes, including oxaloacetate transcarboxylase, biotin-dependent carboxytransferase, CoA transferase, fumarate hydrolase, lactate dehydrogenase, coenzyme B₁₂-dependent methylmalonyl-CoA mutase, malate dehydrogenase, and succinate dehydrogenase.

While most pyruvate is converted to PA/propionate during fermentation, some is converted to acetate. The acetate formation pathway involves intermediates acetyl CoA and acetyl phosphate, and enzymes pyruvate dehydrogenase complex, phosphotransacetylase, and acetate kinase.

A number of carbon sources have been used for microbial PA production, including glucose, fructose, maltose, sucrose, xylose, lactose, glycerol, lactate, flour hydrolysate, molasses, whey, and a combination thereof. A number of culture systems such as batch, fed-batch, and continuous fermentation have been used.

However, for commercial-scale microbial production of organic acids to be economically viable, the fermentation process must be able to convert carbon sources at a high yield (amount of organic acid production from carbon source, typically measured in g/g) and high productivity (rate of organic acid production, typically measured in g/L·h).

Various fermentation technologies, including fed-batch, continuous culture, multi-stage, cell immobilization, and extractive fermentation systems, have been explored to increase the yield of organic acid production. However, the modest increase in yield and productivity often comes is offset by a significant increase in production cost.

For example, coculture methods have been used to produce PA using whey as feedstock (WO 85/04901; EP 0141642 A1). WO 85/04901 describes the use of Lactobacillus casei subspecies rhamnosus in the presence of Veillonella cricetid to interconvert lactate to propionate via a two-stage fermentation process. In the first stage, carbohydrates are converted to lactic acid by L. casei; in the second stage, lactic acid is fermented to PA by V. cricetid. (The genera Lactobacillus and Veillonella both belong to the phylum Firmicutes, whereas the genus Propionibacterium belongs to the phylum Actinobacteria.) EP 0141642 also describes the use of a mixed culture of lactic acid-producing bacteria (L. casei) and PA-producing bacteria (P. shermanii) to maximize the fermentation yield. The coculture systems of WO 85/04901 and EP 0141642 are reported to be very productive in terms of PA production from lactose, with final yields ranging from 20-100 g/L. However, such coculture systems have considerable implications for process parameters. For example, they suffer from a lack of control over the growth and metabolic activity of each member of the system, which can lead to failure of either member to grow or to contribute to formation of the desired product. A lack of reproducibility is common with coculture systems.

One major problem associated with microbial production of organic acids is the strong inhibitory effect of the end product on cell growth and the fermentation process, leading to low production yield and productivity. Acid tolerance was assumed to be crucial to improving the yield and productivity of PA-producing strains (Rehberger and Glatz 1998). The elevated inhibitory effect of PA at pH 4.5-5.0 as compared to lactic acid was attributed to the fact that at this pH range, about half of PA (which has a pKa of about 4.9) would be present in the undissociated, protonated, and uncharged form, whereas lactic acid (which has a pKa of about 3.1) would mostly be in the dissociated, deprotonated, and charged form. It was assumed that because the undissociated acid could penetrate the cell wall and membrane more easily, more PA than lactic acid could get into the cell and exert its inhibitory effect. Enhancement of acid tolerance was thus thought to be an effective strategy to alleviate end-product inhibition and improve PA production. Accordingly, attempts have been made to create “acid tolerant” mutants of propionibacteria under high PA and either uncontrolled or low pH conditions.

For example, adaptive evolution via serial passage has been used to obtain mutant P. acidipropionici with improved acid tolerance (Woskow and Glatz 1991; Zhu et al. 2010). Serial passage is a method of growing microorganisms such as bacteria in two or more iterations in artificial environments, often created in a laboratory setting, to generate spontaneous mutations in the microorganisms as they evolve over the course of the experiment to adapt to one or more new environmental conditions designed for the experiment. For example, repeatedly subjecting microbes to extreme acidic conditions will lead to spontaneous mutations that allow the microbes to adapt to or tolerate such conditions.

In prior work, to create mutations that confer acid-tolerance, the mutant P. acidipropionici strains were adapted to increasing PA concentrations by repeated and serial transfers in selection media containing increasing amounts of PA (from 0.5% to 5% (Woskow and Glatz 1991) or 1.5 g/L to 20 g/L (Zhu et al. 2010)) over a period of one year or longer. Importantly, in these experiments, pH in the selection media having increasing amounts of PA was not controlled, presumably because it was assumed that the inhibitory effects on cell growth and PA production were caused by the acidity of PA.

P. acidipropionici mutant(s) with enhanced PA production has also been obtained by immobilization and adaptation in a fibrous-bed bioreactor (Suwannakham and Yang 2005; Suwannakham 2005). The ability to obtain acid-tolerant mutant(s) in fibrous-bed bioreactor was attributed to the high cell density and viability maintained in the bioreactor and distinct physiology and survivability of immobilized cells as a result of their direct contact with each other and with a solid surface. The higher PA production was attributed in part to higher activity levels of oxaloacetate transcarboxylase and CoA transferase in the mutant(s). Despite the higher PA yield, in the fibrous-bed bioreactor with high cell density, cell growth is limited. Moreover, fibrous-bed bioreactors are expensive and not scalable, and their uses are limited to small-to-medium scale productions.

More recently, random mutagenesis strategies such as genome shuffling have been used to accelerate directed microbial evolution. For example, Guan et al. reported the use of genome shuffling to generate an acid-tolerant mutant P. acidipropionici strain (Guan et al. 2012). To obtain the strain, four successive rounds of genome shuffling via protoplast fusion were performed, and the acid-tolerant strain was selected using media supplemented with increasing amounts of PA (from 5 to 20 g/L). Again, pH in the selection media having increasing amounts of PA was not controlled, presumably because it was assumed that the inhibitory effects on cell growth and PA production were caused by the acidity of PA.

Subsequent analyses identified 24 proteins that significantly differed between the parental and shuffled strains (Guan et al. 2014). The detected proteins were reported to fall into four broad functional classes: cellular metabolism and energy production; DNA replication, RNA synthesis, and translation; posttranslational modification, protein folding, and chaperones; and hypothetical proteins of unknown function.

In another study, genome shuffling was used to generate acid-tolerant mutant P. acidipropionici, P. intermedium, and P. jensenii strains (WO 2017/055932 A2). Three successive rounds of genome shuffling were performed for each set of strains, each followed by selection of colonies from the acidic (pH 3) side of pH/PA gradient plates prepared using agar culture media supplemented with 5 g/L of PA at either pH 3 or pH 6.5. Final individual recombinants were randomly selected after serial dilutions in culture media plates and screened in a 96 well plate containing 100 μl of culture media at pH 5 and 25 g/L of PA. The mutant strains were reported to have enhanced yields of PA relative to native Propionibacterium and other known derivative strains. Genomic analyses of one of the mutant P. acidipropionici strains identified a number of modified genes, including those encoding the ABC polar amino acid transporter, the Cytochrome C biogenesis protein, the ABC multiple sugar transporter, the large subunit of ribosomal RNA, the long chain acyl-CoA synthetase, and the cation diffusion facilitator. In addition, an extra copy of the whole ribosomal RNA gene and an extra copy of the arginine deiminase regulon (ArgR) with a point mutation were found in the mutant strain.

Targeted metabolic engineering of propionibacteria has also been used to increase PA production. These studies generally target enzymes involved in pyruvate metabolism pathways to, for example, either inhibit the acetate formation pathway or enhance the PA formation pathway. For example, Yang and Suwannakham created engineered P. acidipropionici strains with genes encoding acetate kinase (which catalyzes conversion of acetyl phosphate into acetate) and/or phosphotransacetylase (which catalyzes conversion of acetyl CoA into acetyl phosphate) knocked out, with the goal of eliminating or reducing acetate formation and thereby enhancing PA production (US 2011/0151529 A1; Suwannakham 2005).

Yang et al. created engineered P. acidipropionici and P. freudenreichii subsp. shermanii strains transformed with propionyl-CoA:succinate CoA transferase genes to increase PA production by overexpression propionyl-CoA:succinate CoA transferase, which catalyzes conversion of propionyl CoA into propionate (WO 2012/064883 A2). The resulting strains were reported to have increased PA production and resistance to PA, as well as resistance to acidic pH in general. The increased CoA transferase activity is believed to increase carbon flux through the PA formation pathway over the acetate formation pathway.

The table below describes a list of genes that have been manipulated using recombinant DNA. These genes constitute conventional genetic targets where regulatory mutations might be expected to increase PA yields.

TABLE 1 Gene(s) Organism Effect Reference OtsA (trehalose P. acidipropionici Artificially over- Jiang et al. 2015 biosynthesis) expressed Several genes in P. jensenii Artificially over- Guan et al. 2016 arginine deaminase and expressed glutamate decarboxylase systems Propionyl- P. acidipropionici Artificially over- Wang et al. 2015 CoA:succinate CoA P. shermanii expressed WO 2012/064883 A2 transferase Acetate kinase P. acidipropionici Artificial knock out Suwannakham et al. 2006 Suwannakham 2005 US 2011/0151529 A1 Phosphotransacetylase P. acidipropionici Artificial knock out US 2011/0151529 A1

Targeted genetic engineering in propionibacteria, however, is challenging. As an initial matter, the effect of acid alteration and stress on bacterial physiology is complex and not well understood, making it difficult to improve tolerance towards organic acids through manipulation of specific genes. Indeed, despite knowledge about the identity of the intermediates and enzymes in the Wood-Werkman pathway that form PA in propionibacteria, genetic manipulations of the genes in this pathway have not increased PA yields to a significant extent.

Moreover, the high GC content in propionibacteria makes it difficult to identify the locations of individual genes and all of the coding regions in the genome, which complicates genetic manipulation. In addition, there are only a small number of cloning vectors available for introducing recombinant DNA into propionibacteria cells, which are known to have low transformation efficiency. Selection of transformants is also complicated by the ability of propionibacteria to quickly develop spontaneous resistance to antibiotic markers.

In addition to these challenges, the use of recombinant DNA for producing microbial cell lines is incompatible with the development of an organic food ingredient such as PA. At least in the United States, PA or other organic acids produced by genetically engineered microbes cannot be labeled as “organic” or “natural preservative,” which is especially important in the food industry. Therefore, there remains a need for new microbial strains suitable for industrial-scale production of organic acids and methods of making and isolating such strains.

The toxicity of organic acids towards microbes is not well understood despite its relevance in the food and chemical industries that use fermentation for organic acid production. Despite knowledge about the identity of the intermediates and enzymes in the Wood-Werkman pathway that forms PA in propionibacteria, genetic manipulations of the genes in this pathway have not increased PA yields to a significant extent. One reason could be that these genes do not limit PA formation. Therefore, altering their sequence or expression would not change PA levels. Instead, it is argued here that other cellular targets control PA yields, but their identities could not be predicted based on current knowledge. The unknown process is what limits PA formation. Since this process is not known, the genes involved in this process cannot be predicted.

Prior efforts in creating PA-resistant bacteria through serial passage or genome shuffling have generally used media with increasing amounts of PA but either without pH control or at a pH significantly below the pKa of PA. This is based on the idea that toxicity, and therefore resistance, arises from the concentration of the organic acid. However, this approach does not consider the mechanism of organic acid uptake by the cell that involves the transporter system, which depends on the nature of the transporter and the membrane or envelope in which it is located.

Organic acids are weak acids with pKa values generally between about 4-5. The relationship between pH and pKa is described by the Henderson-Hasselbalch equation: pH=pKa+log₁₀([A ⁻]/[HA])

wherein [HA] is the concentration of the protonated, undissociated, and uncharged weak acid, and [A⁻] is the concentration of the deprotonated, dissociated, and negatively charged conjugate base. In a typical fermentation process, the pH of the microbial culture when the organic acid reaches maximum concentration is approximately at the pKa of the organic acid without the use of a buffering agent. A solution having a pH of about 4-5 is not that acidic relative to the known pH tolerance of organic acid producing bacteria. Most of these bacteria do grow at pH values in this range, although the optimum pH for cell growth is typically about 6-7.

Intracellular transport of organic acids can be achieved through diffusion or through the action of membrane transport protein systems depending on whether the organic acids are charged or uncharged. When organic acids are not deprotonated or dissociated, they are uncharged. In this state, they can diffuse across the cellular membranes without reliance on transport systems. Charged molecules, however, always require a transport system to be translocated across membranes.

At a pH value that equals its pKa value, half of the organic acid is in the protonated (or undissociated), uncharged form, while the other half is in the deprotonated (or dissociated), negatively charged form. At pH values below their pKa values, organic acids would mostly be uncharged because their carboxyl groups would be protonated. At pH values above their pKa values, organic acids would mostly be unprotonated or dissociated and therefore negatively charged.

At high concentrations of the organic acid, the pH is relatively low, and the organic acid would mostly be in the uncharged state and could diffuse into the cell in its acid form. This is the basis for prior efforts to isolate organic acid resistant microbes either without pH control or at a pH significantly below the pKa of the organic acid. The approach in theory would generate cell lines with mutations that produced resistance due to diffusion-based organic acid cell entry. It was assumed that the uncharged organic acid would diffuse through the cell membrane into the cytoplasm and release protons due to the relatively alkaline pH inside the cell; the increase in intracellular acidity would inhibit cell growth and organic acid formation. In other words, it was assumed that organic acids in their uncharged state limited their own production. Despite the published literature and patents, in our experience, this approach does not generate resistant microbes effectively, and may require years of passage to work.

We hypothesized that it was not the acidity of the organic acid that was toxic, as previously assumed by others. Rather, it was the deprotonated, negatively charged form or the neutral salt of the organic acid (propionate) that was toxic, and would be more effective as a selection agent to recover spontaneous resistance mutations.

Unlike prior efforts, we hypothesized that the use of pH control at a value above the pKa value of the organic acids to be produced, and preferable at least 1 unit above, would ensure that most of the organic acids remain in a charged and deprotonated form. In this form, they would remain dependent on protein transport systems for intracellular uptake. This would avoid recovery of cell lines with mutations that produced resistance due to diffusion-based organic acid cell entry, if such mutations could be discovered.

Specifically, the process used was serial passage of the starting microbial cell line (usually but not necessarily a wild-type) in free-cell (i.e. non-immobilized or planktonic) culture in a bacteriologic culture medium supplemented with organic acid of interest in an amount that is sufficient to inhibit normal microbial growth (either in progressively increasing amounts or the same amount for all passages) under conditions of continued pH control at a specific pH that is above the pKa value of the organic acid. The pH is controlled at a value above the pKa value of the organic acid, preferably in the range between about 5.5 and about 7.5, more preferably at or near neutral, between about 6.0 and about 7.0, and most preferably at about 7.0. Although the present examples describe the use of Propionibacterium, other microbes can be used that are fermentative organisms that excrete organic acids, e.g., Lactobacillus, Acetobacter, Gluconobacter, or Clostridium. The organic acid used can be, e.g., PA, lactic acid, acetic acid, or butyric acid. In some embodiments, the microbe is from the genus Propionibacterium (Acidipropionibacterium), and more preferably the species P. acidipropionici, and the organic acid is PA. In some embodiments, the microbe is from the genus Lactobacillus, and more preferably the species L. acidophilus, and the organic acid is lactic acid. In some embodiments, the microbe is from the genus Acetobacter or the genus Gluconobacter, and the organic acid is acetic acid. In some embodiments, the microbe is from the genus Clostridium, and more preferably the species C. butyricum, and the organic acid is butyric acid.

Using our method of serial passage with pH control, we were able to create and isolate a new microbial strain having increased organic acid production compared to the parental strain in less than two weeks, much faster than using the conventional serial passage method described in Woskow and Glatz 1991, which generally takes at least one year. Our method is also much less complex and more easily scalable than other random mutagenesis methods such as genome shuffling and cell immobilization in a fibrous-bed bioreactor or targeted genetic engineering. Organic acids produced by mutant cell lines created and isolated using serial passage with pH control can be labeled as “organic” or “natural preservative,” which is especially important in the food industry.

The same method of serial passage with pH control can be used to make and isolate a variety of microbes, including but not limited to propionibacteria, lactobacilli, acetic acid bacteria, and clostridia, that overproduce a number of organic acids, including but not limited to PA, lactic acid, acetic acid, and butyric acid. All charged molecules depend on transport systems and their associated membranes/envelopes for function. Alterations in these cellular components would achieve the same outcome as described here for propionate for other organic acids.

The same selection method (i.e., using bacteriologic culture medium supplemented with organic acid of interest in an amount that is sufficient to inhibit normal microbial growth under conditions of continued pH control at a pH that is above the pKa value of the organic acid) can be used in screening microbial libraries generated from genome shuffling or other random mutagenesis methods for isolates that exhibit increased organic acid tolerance and production.

Using this pH control method, we were able to target unique mechanisms for resistance that depended on transport and/or unpredictable intracellular targets including those involved in regulation and metabolism. Genome resequencing was then used to identify the critical genes through their mutational changes that caused the genetic resistance to high concentrations of organic acids.

The resulting mutations generally affected cellular envelope functions, as shown in Table 2.

TABLE 2 ENVELOPE AND ASSOCIATED CATEGORIES ENVELOPE FUNCTIONS: Transporters/membrane proteins (10 affected ORFS): Major facilitator superfamily proteins, amino acid permeases, hypothetical membrane protein, LemA membrane protein, intramembrane metalloprotease, AAA ATPase, sodium-proton antiporter Gain-of-function in penicillin-binding protein and amino acid permease Cell wall/peptidoglycan synthesis: Penicillin-binding proteins, O-antigen ligase domain- containing proteins (many mutations) ENVELOPE MODIFYING FUNCTIONS: Oxidation/reduction: Flavin reductase, alpha/beta hydrolase, pyruvate carboxylase, MocA oxidoreductase, protophyringen oxidase, KGD Glycosyl transferases/hydrolases: Glycosyl transferase, glycosyl hydrolase, adenine glycosylase

These mutations primarily altered the structure and composition and function of the cellular envelope, which consists of the cell wall and membrane(s), including the cytoplasmic membrane. A complete list of the mutations identified is provided in Table 3. We did not see any mutations in genes that have been targeted for metabolic engineering and manipulated using recombinant DNA as previously reported (see Table 1). Mutations in multiple genes appear to be required to produce the mutant phenotype (such as increased growth in media supplemented with organic acid and/or overproduction of organic acid compared to the starting microbial cell line). This is in direct contrast to prior knowledge where single genes were manipulated to try to change PA yields.

In accordance with the present invention, other conventional microbiology, molecular biology, recombinant DNA, and biochemical techniques may be used. Such techniques are fully explained in the literature and within the skill of the art. The invention will be further described in the following examples, which do not limit the scope of the methods and compositions of matter recited in the claims.

EXAMPLES Example 1

Isolation of Strain 3-1

A P. acidipropionici (ATCC 25562) was grown to high cell density in 10 mL M24+2.0% glucose media. Serial dilutions of this culture (10⁰ to 10⁻³) were then plated on solid M24+2.0% glucose media, solidified with agar, supplemented with 1.0%, 2.0%, and 3.0% (w/v) PA, all neutralized to pH 7.0 using sodium hydroxide. Cells were also plated on solid M24+2.0% glucose media with no additional PA.

After a 5-day anaerobic incubation at 30° C., colony growth at the different PA concentrations was assessed. Three colonies grew on the 3% PA plate plated with undiluted cells; no colony grew on the 3% PA plates plated with diluted cells. The three colonies were isolated and re-streaked onto no-PA, 2.0% PA, and 3.0% PA plates (all neutralized to pH 7.0 using sodium hydroxide), along with freshly grown wild-type P. acidipropionici cells.

After a second 5-day anaerobic incubation at 30° C., colony growth at the different PA concentrations was again assessed. All three isolates, but not wild-type, were able to grow on the 1.0% PA plate (FIG. 1 ). Only isolate #1 was able to grow on the 2.0% PA and 3.0% PA plates. This isolate was named strain 3-1 (“3” denotes 3.0% PA, and “1” denotes isolate #1). Isolate #1 was inoculated into 5 mL liquid M24+2.0% glucose media and grown to high cell density, and frozen permanents of these cells were made.

After the phenotype of resistance to 3.0% PA on solid media was confirmed for strain 3-1, PA production in 10 mL batch cultures and 1 L bioreactor cultures of this strain was compared to its parental P. acidipropionici (ATCC 25562) cells by HPLC in a broad range of media and cultivation conditions.

Strain 3-1 was deposited under the name NFS-2018 on Jul. 10, 2019, in the American Type Culture Collection (10801 University Blvd. Manassas, Va. 20110-2209) and assigned Accession Number ATCC PTA-125895).

Example 2

PA Production by Strain 3-1 and Wild-Type P. acidipropionici

Wild type P. acidipropionici (ATCC 255562) and strain 3-1 were cultivated from a frozen permanent at 30° C. under anaerobic condition in M24 medium supplemented with 2% glucose. The cells were sub-cultured every 48 hr into fresh M24 medium starting at 10 mL then at 50 mL to use as seed for the 1 L bioreactor vessels.

For preparation of wheat flour medium, 75 g of American cake flour was added to 1 L of ddH2O in a sterile 2 L flask while mixing. One mL of Enzenco alpha-amylase and 500 mL of 50 ppm of CaCl2 was added to the mixture to hydrolyze the cake flour. The pH was adjusted to 6.0 by adding 5 mL of 5M NaOH and the temperature was held at 90° C. for 1 hour. The mixture was allowed to cool then incubated at 37° C. overnight. After the overnight incubation, the temperature was raised to 60° C. and pH adjusted to 7.0 by adding 2 mL of 5M NaOH. To release glucose, 1 mL of Enzenco glucoamylase, 0.05 g of protease, 0.4 g of MgSO4, and 10 g of Ohly KAT yeast extract were added to the mixture while stirring. The mixture was held at 60° C. for 2 hours. The mixture was allowed to cool then added to a glass-jacketed bioreactor vessel then sealed. Before autoclaving, the pH was calibrated.

Fermentations were performed at 1 L volumes in the 3 L bioreactor vessels. The temperature was maintained at 30° C., the pH was maintained at 7.0 using 5M NaOH, and cultures were agitated at 200 rpm. 3 mL of filtered sterile trace element solution was added to the bioreactor before inoculation. The glucose concentration was determined using a YSI 2900 analyzer. The 1 L of wheat flour medium was seeded with 5% inoculum. Samples were removed every 24 hours for PA analysis on the HPLC. The results are shown in FIG. 2 .

Both strain 3-1 and the parental wild-type strain reached maximum PA concentration at about 120 hours. The maximum concentration of PA produced by strain 3-1 is about 36 g/L, compared to about 30 g/L by the parental wild-type strain.

Additional experiments were carried out under 5-6 different conditions, 3-4 times each, to compare PA production by strain 3-1 and wild-type P. acidipropionici. Results similar to those shown in FIG. 2 were obtained. There is a minimum of 15% increase in PA production by strain 3-1 compared to the wild-type after 60 hours of culturing.

Example 3

Genomic Analyses of Strain 3-1

Genome resequencing of strain 3-1 was used to identify the critical genes through their mutational changes that caused the genetic resistance to high concentrations of organic acids.

65 loss of function mutations in 29 genes were identified. The mutations generally affected cellular envelope functions, as shown in Table 2. These mutations primarily alter the structure and composition and function of the cellular envelope, which consists of the cell wall and membrane(s), including the cytoplasmic membrane. A complete list of the mutations identified in strain 3-1 is provided in Table 3.

TABLE 3 STRAIN 3-1 GENOME MUTATIONS SEQ ID Coordinates ORF NO. Change Non- synonymous 130744- ASQ49_RS00690 1 Arg → His 130746 class I SAM-dependent methyltransferase 130744- ASQ49_RS00695 2 Thr → Pro 130746 MFS transporter 130748 ASQ49_RS00690 1 Pro → Leu class I SAM-dependent methyltransferase 130752 ASQ49_RS00695 2 Arg → Gly MFS transporter 181601 ASQ49_RS00915 3 Insertion (no (80% LemA family protein frameshift) confidence) 181607- ASQ49_RS00915 3 Gln → Leu 181609 LemA family protein 240311 ASQ49_01155 4 Pro → His Flavin reductase 240440 ASQ49_01155 4 Ala → Val Flavin reductase 281222 ASQ49_RS01330 5 Trp → STOP Hypothetical protein (BLAST hit to MFS transporter) 344598 ASQ49_RS01635 6 Thr → Pro MFS transporter 525954 ASQ49_RS02385 7 Ala → Glu glycosyl transferase family 1 548143- ASQ49_RS02475 8 Ala → Gly 548147 Hypothetical protein Gly → Leu (Strong BLAST hits to O-antigen ligase and membrane protein) 548153- ASQ49_RS02475 8 Ala → Val 548156 Hypothetical protein Gly → Leu (Strong BLAST hits to O-antigen ligase and membrane protein) 548162 ASQ49_RS02475 8 Ala → Val Hypothetical protein (Strong BLAST hits to O-antigen ligase and membrane protein) 558158- ASQ49_RS02520 9 His → Gly 558160 O-antigen ligase domain-containing protein 558181 ASQ49_RS02520 9 Gln → His O-antigen ligase domain-containing protein 558228 ASQ49_RS02520 9 Ala → Thr O-antigen ligase domain-containing protein 558252 ASQ49_RS02520 9 Ser → Pro O-antigen ligase domain-containing protein 558258 ASQ49_RS02520 9 Ser → Pro O-antigen ligase domain-containing protein 558266 ASQ49_RS02520 9 Arg → Leu O-antigen ligase domain-containing protein 558273 ASQ49_RS02520 9 Ser → Ala O-antigen ligase domain-containing protein 558279 ASQ49_RS02520 9 Gln → Glu O-antigen ligase domain-containing protein 558282 ASQ49_RS02520 9 Glu → Gln O-antigen ligase domain-containing protein 558288- ASQ49_RS02520 9 Leu → Pro 588290 O-antigen ligase domain-containing protein 558291- ASQ49_RS02520 9 Glu → Val 558293 O-antigen ligase domain-containing protein 558306 ASQ49_RS02520 9 Pro → Ala O-antigen ligase domain-containing protein 558308- ASQ49_RS02520 9 Thr → Ser 558310 O-antigen ligase domain-containing protein 562835 ASQ49_RS02535 10 Val → Leu O-antigen ligase domain-containing protein 562840 ASQ49_RS02535 10 Ala → Val O-antigen ligase domain-containing protein 562843 ASQ49_RS02535 10 Gly → Ala O-antigen ligase domain-containing protein 566353 ASQ49_RS02550 11 Lys → Gln (*50% penicillin-binding frequency) protein 566356 ASQ49_RS02550 11 Ala → Ser (*50% penicillin-binding frequency) protein 618017- ASQ49_RS02820 12 Glu → Ala 618019 Phosphotransferase 738302 ASQ49_RS03340 13 Thr → Ala Alpha/beta hydrolase 742073 ASQ49_RS03360 14 Ile → Leu Hypothetical protein (BLAST hits to intramembrane metalloprotease) 1176596 ASQ49_RS05220 15 Ala → Val gfo/Idh/MocA family oxidoreductase 1279986 ASQ49_RS05630 16 Ile → Val Alpha/beta hydrolase 1331356 ASQ49_RS05840 17 Gly → Ser Amino acid permease 1331366 ASQ49_RS05840 17 Arg → His Amino acid permease 1521847 ASQ49_RS06625 18 Thr → Ala Hypothetical protein (BLAST hits to protoporphyrinogen oxidase) 1816621 ASQ49_RS07985 19 Ser → Ala Adenine glycosylase 1816687 ASQ49_RS07985 19 In-frame Adenine glycosylase insertion (1 amino acid) 1817191 ASQ49_RS07985 19 Gly → Glu Adenine glycosylase 1817202 ASQ49_RS07985 19 Glu → Ala Adenine glycosylase 1854503 SQ49_RS08150 20 Lys → Arg Hypothetical protein (BLAST hit to sodium-proton antiporter) 1854520 SQ49_RS08150 20 Ile → Met Hypothetical protein (BLAST hit to sodium-proton antiporter) 2679601 ASQ49_12020 21 His → Asp multifunctional oxoglutarate decarboxylase/ oxoglutarate dehydrogenase thiamine pyrophosphate- binding subunit/ dihydrolipoyllysine- residue succinyltransferase subunit (kgd) 2927020 ASQ49_RS13125 22 In-frame Amino acid permease insertion (4 amino acids) 2927030 ASQ49_RS13125 22 Gly-Ser → Amino acid permease Ala-Ala 2928883 ASQ49_RS13130 23 Asn → Tyr Hypothetical protein (glycosyl gydrolase family) 3517645 ASQ49_RS15965 24 Thr → Arg (*50% M18 family frequency) aminopeptidase 3517646 ASQ49_RS15965 24 Thr → Ser (*50% M18 family frequency) aminopeptidase 3517648 ASQ49_RS15965 24 Ser → Thr (*50% M18 family frequency) aminopeptidase 3517649 ASQ49_RS15965 24 Ser → Gly (*50% M18 family frequency) aminopeptidase 3517652 ASQ49_RS15965 24 Ser → Tyr M18 family aminopeptidase 3517655 ASQ49_RS15965 24 Ser → Asn M18 family aminopeptidase FRAMESHIFTS 558244 ASQ49_RS02520 9 O-antigen ligase domain-containing protein 558246 ASQ49_RS02520 9 O-antigen ligase domain-containing protein 2867178 ASQ49_RS12835 25 AAA ATPase FRAMESHIFT REPAIRS 448285 ASQ49_RS02075 26 DUF1116 domain- containing protein 561527 ASQ49_JRS02530 27 Glycosyl transferase 900222 ASQ49_RS03980 28 acetyl-CoA carboxylase biotin carboxyl carrier protein subunit 919056 ASQ49_RS04070 29 Penicillin-binding protein 1330401 ASQ49_RS05840 17 Amino acid permease 1330407 ASQ49_RS05840 17 Amino acid permease

Mutations in these genes (or their homologues in other species described herein) likely confer genetic resistance to high concentrations of organic acids by altering the membrane transport protein systems and/or previously unknown intracellular targets involved in regulation and/or metabolism.

Multiple mutations in the same gene imply that the gene is very important for the trait and required multiple changes to contribute to the trait. Noticeably, several genes had three or more mutations, which may indicate their critical roles in limiting organic acid formation. They include genes encoding: O-antigen ligase domain-containing protein (15 mutations in ASQ49_RS02520; 3 mutations in ASQ49_RS02535; and 3 mutations in ASQ49_RS02475 (hypothetical protein with strong BLAST hits to O-antigen ligase and membrane protein)); M18 family aminopeptidase (6 mutations in ASQ49_RS15965); amino acid permease (4 mutations in ASQ49_RS05840); and adenine glycosylase (4 mutations in ASQ49_RS07985).

REFERENCES

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The invention claimed is:
 1. A Propionibacterium acidipropionici (P. acidipropionici) cell comprising one or more of the loss of function mutations in genes encoding DUF1116 domain-containing protein (A5Q49_R502075; SEQ ID NO:26) and M18 family aminopeptidase (A5Q49_RS15965; SEQ ID NO:24) identified in Table
 3. 2. The P. acidipropionici cell of claim 1 further comprising one or more additional mutations identified in Table
 3. 3. The P. acidipropionici cell of claim 1, wherein the cell comprises a loss of function mutation in each of the genes encoding the proteins identified in Table
 3. 4. A P. acidipropionici cell comprising at least one loss of function mutation in gene(s) encoding protein(s) selected from the group consisting of O-antigen ligase domain-containing protein (ASQ49_RS02520; SEQ ID NO:9), amino acid permease (ASQ49_RS13125; SEQ ID NO:17), DUF1116 domain-containing protein (A5Q49_R502075; SEQ ID NO:26) and M18 family aminopeptidase (A5Q49_RS15965; SEQ ID NO:24), and adenine glycosylase (ASQ49_RS07985; SEQ ID NO:19).
 5. The P. acidipropionici cell of claim 4 comprising a loss of function mutation in the gene encoding O-antigen ligase domain-containing protein (ASQ49_RS02520; SEQ ID NO:9). 